The present invention relates to a focus detecting apparatus, and more particularly, to a focus detecting method and its apparatus which detect a focus condition in an optical apparatus such as a camera, a microscope or the like.
There have been known two methods of detecting a focus condition of an object image formed by an image forming lens, one being the so-called image blur detecting methods which determine an evaluation value which indicates the sharpness of an image projected onto photoelectric transducer means and the other being the so-called image shift detecting methods which determine an evaluation value which indicates shift or displacement between two images projected onto photoelectric transducer means by two light beams passing primarily through one half and another half portions of an exit pupil which are divided by a plane including an optical axis of an image forming lens.
Such conventional methods will be described hereinafter with reference to the attached drawings. FIG. 1 is a schematic sectional view of an optical system of a focus detecting apparatus embodying the image blur detecting method which is applied to a single-lens reflex camera. In FIG. 1, a light beam through a taking lens 1 is reflected by a quick-return mirror 3 one part of which is formed as a half-mirror 3a and is led to a view finder optical system (not shown) located in the upper side of the camera. A light beam passing through the half-mirror 3a is directed further to a beam splitter 6 of a focus detecting apparatus 5 located in the lower side of the camera by a total reflection mirror 4 which is disposed behind the mirror 3. The beam splitter 6 has two optical path separating planes so that the incident light beam is divided into two optical paths and respective light beams impinge on photoelectric transducer element rows 8, 9 on a light receiving substrate 7 located below beam splitter 6. The transducer element rows 8, 9 have light receiving surfaces whose positions are optically equivalent to plane positions 11, 12 at a given distance interval of .DELTA. before and behind a presumed focal plane (a film surface) 10 of the lens 1, respectively. In the focus detecting apparatus, the sharpness of images on light receiving surfaces of the rows 8, 9 are obtained by outputs of respective rows and focus conditions for a front focus, rear focus and in focus are determined by comparing the sharpness values thus obtained of two images.
Generally, a light intensity P of a point image formed by an image forming optical system is given by P.varies.F.sup.2 /dZ.sup.2, where F is an F number of a lens and dZ is an amount out of focus. From the expression, as shown in FIG. 2, the intensity P attenuates rapidly when an out of focus condition occurs and the rate of change in the intensity extremely decreases when the amount out of focus increases beyond a certain value. Accordingly, even though the apparatus can detect an in-focus condition with a high accuracy, the difference between the intensities of images detected by the rows 8, 9 decreases when the amount out of focus is large. As a result, even though the lens 1 is largely out of position from the focal plane 10 in a front focus or a rear focus, the situation may be erroneously determined as an in-focus. To eliminate this disadvantage, it is conceivable that the given distance .DELTA. in the optical path is made larger so that the difference between the light intensities may increase when the amount out of focus becomes large. In such case, the light intensities in the transducer element rows 8, 9 for an image only having high frequency components are extremely reduced when properly in-focus and the difference between the light intensities in the rows 8, 9 does not vary sufficiently within a wide defocused range so that the in-focus detection may be difficult. Conversely, when the distance .DELTA. in the optical path is reduced in accordance with an image including high frequency components, an in-focus condition for the image including high frequency components may be detected with a high accuracy. However, in this case, as described above, there is a possibility of erroneously determining an in-focus condition when the amount out of focus is large and it is difficult to detect an in-focus condition for an image only having low frequency components since the difference between their light intensities is insufficient to detect. In addition, in such an image blur detecting method, the difference between light intensities does not directly correspond to the defocused amount from the focal plane 10 of the lens 1 so that the amount of shift or displacement of the lens 1 to the in-focus position may not be determined.
FIG. 3 is a schematic section view of an optical system in a focus detecting apparatus embodying the image shift detecting method which is applied to a single-lens reflex camera and similar parts to those in FIG. 1 are given like reference numerals. In the focus detecting apparatus 15, a light beam from a taking lens 1 which is reflected by a total reflection mirror 4 impinges through a fly-eye lens array 16 which is formed with a number of fly-eye lenses onto a photoelectric transducer element row 18 on a light receiving substrate 17 which is disposed proximate a plane which is optically equivalent to the presumed focal plane 10 and in close proximity to the lens array 16. The transducer element row 18, as further shown in FIG. 4, is composed of two groups of photoelectric transducer elements 18A, 18B, each group having n pieces of transducer elements. All transducer elements 18A.sub.1 to 18A.sub.n and 18B.sub.1 to 18B.sub.n of the element groups 18A, 18B are arranged in a straight line and the two adjacent elements in each of the groups 18A, 18B form a pair. The fly-eye lens array 16 has n fly-eye lenses 16.sub.1 to 16.sub.n corresponding to n pairs of photoelectric transducer elements and is arranged in such a manner that two transducer elements forming each pair of transducer elements separately receive light beams which pass primarily through mutually different portions of an exit pupil of the lens 1, an upper and a lower portion divided by a plane including an optical axis 2 in FIG. 3 which plane is perpendicular to the sheet surface.
In the apparatus 15 constructed as described above, when an image of an object being photographed is projected through the lens 1 and the lens array 16 onto the transducer element row 18, a light beam passing primarily through the upper portion in the exit pupil of the lens 1 impinges on the transducer element group 18A and a light beam passing primarily through the lower portion in the exit pupil of the lens 1 impinges on the transducer element group 18B. The light intensity distributions of the image projected onto the transducer element groups 18A, 18B coincide when in-focus and displace with respect to each other in the opposite directions when out of focus, as shown in FIG. 5, in accordance with respective displacement directions. The focus detecting apparatus 15 detects the displacement direction of the image by properly processing outputs of the transducer element groups 18A, 18B and determines a focussing condition, such as a front focus, a rear focus or an in-focus, based on the detection of the displacement direction.
The focus detecting apparatus employing the image shift detecting method has much greater advantages in a detectable range of focus than the image blur detecting method. However, the apparatus requires the provision of a gap between successive fly-eye lenses 16.sub.1 to 16.sub.n so that the images of the exit pupil of the lens 1 which are formed by each of fly-eye lenses 16.sub.1 to 16.sub.n do not overlap each other. Accordingly, when the light intensity of an image has particularly high frequency components which change rapidly in a step form, for example, and when the step portion lies within a gap between successive fly-eye lenses, the gap belongs to an insensible zone for the image and hence the focusing accuracy will decrease. As a result, in the case of an image having high frequency components, a position which is determined as bring an in-focus position varies in accordance with a relative position between the image and each fly-eye lens and the accuracy will be insufficient. To solve the disadvantages, it is conceivable that the diameter of each fly-eye lens is further reduced or the focal length of each fly-eye lens is further shortened so as to reduce the gap between successive fly-eye lenses. With such construction, an available amount of light decreases to deteriorate the sensitivity and it is difficult to process a fly-eye lens and to make an optical adjustment between each fly-eye lens and each pair of transducer elements. Furthermore, an apparatus with such construction can detect the amount of an image shift or displacement but when the F number of the lens 1 changes by interchanging with another lens, it may cause the case that the defocused amount from the presumed focal plane 10 of the lens is not determined unless information regarding the F number of the lens 1 is given previously in any form. Thus, it is difficult to determine the amount of shift to position the lens 1 in an in-focus.
As described above, in the image blur detecting method, the range in which each of the focus conditions of a front focus, a rear focus and an in-focus is accurately detectable is limited but an in-focus condition for an image particularly having high frequency components can be advantageously detected with high accuracy by comparatively reducing the difference in the optical path between two transducer element rows disposed on opposite sides of the presumed focal plane 10. In the image shift detecting method, the range where a focus is detectable is wide and an in-focus condition for an image particularly having low frequency components can be advantageously detected with a high accuracy without practically causing the above-mentioned insensible zone for the image shift. However, in the image blur detecting method, since the difference in sharpness which is determined by transducer element rows does not directly correspond to the defocused amount from the presumed focal plane of the lens, the amount of movement of the lens to the in-focus position is not determined. In the image shift detecting method, although the amount of image shift can be detected, the amount of moving the lens to the in-focus position from the expected focal plane can not be determined unless the F number of the lens is previously given in any form. Accordingly, it is to be noted that the conventional focus detecting apparatus determines consecutively an in-focus condition while in detecting a focus condition and hence adjustment in an in-focus may not be rapidly conducted.